Lithium secondary batteries (e.g., a lithium ion battery and a lithium polymer battery), which have a high energy density, have been used as a main power supply for, for example, mobile communication devices or portable electronic devices. In addition, such a lithium secondary battery has become of interest as a large-scale power supply for electricity storage or a vehicle power supply.
Hitherto, the anode of such a lithium secondary battery has generally been formed of any carbon material, such as graphite or low-crystallinity carbon. However, the anode formed of such a carbon material exhibits low available current density and insufficient theoretical capacity. For example, graphite, which is a type of carbon material, has a theoretical capacity as low as 372 mAh/g. Thus, demand has arisen for development of an anode having a higher capacity.
Meanwhile, as has been known, when an anode formed of metallic Li is employed in a lithium secondary battery, high theoretical capacity is achieved. However, such a battery poses a critical problem in that dendritic metallic Li is deposited on the anode during charging, and the dendrite is grown through repeated charging/discharging and reaches the positive electrode (i.e., a cathode), resulting in internal short-circuit. In addition, since the thus-deposited dendritic metallic Li has large specific surface area and thus high reaction activity, an interfacial film is formed on the surface of the dendrite from a decomposition product of a solvent having no electron conductivity, whereby the internal resistance of the battery increases, resulting in reduction of charging/discharging efficiency. Thus, a lithium secondary battery including an anode formed of metallic Li exhibits low reliability and has short cycle life. Therefore, such a lithium secondary battery has not been widely put into practice.
Under these circumstances, demand has arisen for an anode active material formed of a material, other than metallic Li, which has a discharge capacity greater than that of a generally used carbon material. As has been known, for example, an element such as Sn, Si, or Ag, or a nitride or oxide of such an element can occlude Li ions and can form an alloy with Li ions, and the amount of Li ions occluded therein is considerably greater than that of Li ions occluded in any carbon material.
However, in the case where an anode formed of, for example, an element such as Sn, Si, or Ag, or a nitride or oxide of such an element is employed in a lithium secondary battery, when the battery is subjected to repeated charging/discharging cycles, considerable expansion and contraction of the anode may occur in association with occlusion and release of Li ions, and the expansion and contraction may cause cracking or disintegration of the anode. Therefore, a lithium secondary battery including an anode formed of the aforementioned substance (e.g., an element such as Sn, Si, or Ag, or a nitride or oxide of such an element) exhibits reduced cycle life, and thus cannot be used as a practical battery.
In order to solve such a problem, there has been proposed an anode active material formed of an alloy having two or more phases, the alloy containing a metal which is likely to occlude and release Li ions, and a metal which neither occludes nor releases Li ions, wherein the latter metal is incorporated for the purpose of preventing expansion and contraction of the anode during occlusion and release of Li ions, as well as cracking or disintegration of the anode caused by expansion and contraction of the anode.
For example, Patent Document 1 describes an anode active material containing an Li-ion-occluding phase α, and a phase β formed of an intermetallic compound or solid solution of an element forming the Li-ion-occluding phase α and another element, the anode active material having a structure produced by rapid solidification of a molten raw material having a selected composition through, for example, the atomization method or the chill roll method. Meanwhile, Patent Document 2 describes an anode active material formed of composite powder produced by mixing the following raw materials: component A, which is at least one element selected from the group consisting of Ag, Al, Au, Ca, Cu, Fe, In, Mg, Pd, Pt, Y, Zn, Ti, V, Cr, Mn, Co, Ni, Y, Zr, Nb, Mo, Hf, Ta, W, and rare earth elements, and component B, which is at least one element selected from the group consisting of Ga, Ge, Sb, Si, and Sn, and by subjecting the resultant mixture to mechanical alloying treatment.
Although an anode formed from the anode active material described in Patent Document 1 or 2 exhibits high initial discharge capacity, there cannot be effectively prevented expansion and contraction of the anode through repeated charging/discharging, as well as cracking or disintegration of the anode caused by expansion and contraction of the anode. Thus, the anode active material has not yet realized prolongation of the cycle life of a lithium secondary battery.